The invention is generally related to a self-charging (e.g., energy harvesting) sensor that wirelessly transmits signals corresponding to measured quantities. More specifically, the invention is related to a sensor that harvests energy from the same quantity (e.g., activity, temperature, flow, moisture, etc.) that is being measured by the sensor such that a change in the rate of charge of the energy harvesting results in a change in the frequency of reporting thereby providing measurement information regarding the measured quantity.
Conventional energy harvesting mechanisms are well known. One example of energy harvesting may include perpetual watch movements that generate and store mechanical energy based on movement of the wearer. An off center weight or cam may rotate on a spindle based on movement in the arm or hand of the wearer. The rotation of the cam may be translated into stored energy by passing a magnet by a coil that is coupled through a diode to capacitor. Another example is thermal energy harvesting that uses thermoelectric materials to generate an electrical potential from the heat flux passing through the thermocouple elements, with the electrical potential used to charge a capacitor. Another example of energy harvesting is a flow meter that may be equipped with permanent magnets on a rotating plate. As the magnets rotate with the plate during flow of the measured liquid, a current may be induced into stators generating energy with which the meter may be powered.
Wireless sensors are also known, but may require relatively complex sensing and transmission modules along with batteries or power sources that correspondingly require a large sensor footprint. The complex circuitry may further lead to rapid battery power consumption. For applications that require smaller form factor sensors, such as medical applications, energy harvesting becomes a greater challenge as the need for power that may be necessary to drive the sensor and provide information about the measured quantity may exceed the relatively small amount of energy generated by the energy harvesting mechanism. Also, to be practical as a disposable medical sensor, sensor elements must be simple and inexpensive to manufacture.